1. Field of the Invention
The present invention relates to cellular mobile communication networks, for example Code Division Multiple Access (CDMA) cellular networks.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings shows parts of a cellular mobile telecommunication network according to the Telecommunication Industries Association (TIA)/Electronic Industries Association (EIA) Standard TIA/EIA/IS-95 of October 1994 (hereinafter xe2x80x9cIS95xe2x80x9d). Each of three base transceiver stations (BTSs) 4 (BTS1, BTS2 and BTS3) is connected via a fixed network 5 to a base station controller (BSC) 6, which is in turn connected to a mobile switching center (MSC) 7. The BSC 6 serves to manage the radio resources of its connected BTSs 4, for example by performing hand-off and allocating radio channels. The MSC 7 serves to provide switching functions and coordinates location registration and call delivery.
Each BTS 4 serves a cell 8. When a mobile station (MS) 10 is in a so-called xe2x80x9csoft hand-offxe2x80x9d (SHO) region 9 where two or more cells overlap, a mobile station can receive transmission signals (downlink signals) of comparable strength and quality from the respective BTSs of the overlapping cells. Transmission signals (uplink signals) produced by the mobile station (MS) can also be received at comparable strengths and qualities by these different BTSs when the mobile station is in the SHO region 9.
FIG. 2 of the accompanying drawings shows a situation where the MS 10 is located within the SHO region 9, and is transmitting such uplink signals that are being received by plural BTSs 4. According to the IS95 standard, a BTS 4 that receives such an uplink signal from the MS 10 relays the signal to the BSC 6 via a dedicated connection line of the fixed network 5. At the BSC 6, one of the relayed signals is selected based on a comparison of the quality of each of the received signals, and the selected signal is relayed to the MSC 7. This selection is referred to as Selection Diversity.
Similarly, FIG. 3 of the accompanying drawings shows a situation where the MS 10 is located within the SHO region 9 and is receiving downlink signals from plural BTSs 4. According to the IS95 standard, downlink signals received by the BSC 6 from the MSC 7 are relayed to all BTSs 4 involved in the soft hand-off via respective connection lines of the fixed network 5, and subsequently transmitted by all the BTSs 4 to the MS 10. At the MS 10 the multiple signals may be combined, for example, by using maximum ratio combination (MRC), or one of them may be selected based on the signal strength or quality, i.e. using Selection Diversity as for the uplink case.
In contrast to, for example, Global System for Mobile Communication (GSM) networks, in CDMA networks each BTS 4 transmits at the same frequency. Consequently, careful control of transmission power must be maintained to minimize interference problems.
Signals are transmitted as a succession of frames according to the IS95 standard. As FIG. 4 of the accompanying drawings shows, each frame is of duration 20 ms, and comprises sixteen 1.25 ms time slots. In each time slot several bits of user data and/or control information can be transmitted.
Power control of transmissions from the MS 10 to the BTSs 4 (uplink power control) in IS95 is achieved as follows. When a BTS 4 receives a signal from the MS 10 it determines whether a predetermined property of the received signal (for example absolute signal level, signal to noise ratio (SNR), signal-to-interference ratio (SIR), bit error rate (BER) or frame error rate (FER)) exceeds a pre-selected threshold level. Based on this determination, the BTS 4 instructs the MS 10 either to reduce or to increase its transmission power in the next time slot.
For this purpose, two bits in every time slot of a pilot channel (PCH) from the BTS 4 to the MS 10 are allocated for uplink power control (see FIG. 4). Both bits have the same value, and accordingly will be referred to hereinafter as the xe2x80x9cpower control bitxe2x80x9d (or PCB) in the singular. The power control bit is assigned a value of zero by the BTS 4 if the MS 10 is required to increase transmission power by 1 dB, and a value of one if the MS 10 is required to decrease transmission power by 1 dB. The BTS 4 is not able to request directly that the MS 10 maintain the same transmission power; only by alternately transmitting ones and zeros in the power control bit is the transmission power maintained at the same level.
When the MS 10 is in a SHO region 9, the MS 10 is required to make a decision on whether to increase or to decrease uplink transmission power based on a plurality of power control bits received respectively from the BTSs 4 involved in the soft hand-off. Consequently, an OR function is performed on all the power control bits. If the result of this OR function is zero then the MS 10 will increase power on uplink transmissions, and if the result is one then the MS 10 will decrease power on uplink transmissions. In this way, uplink transmission power is only increased if all BTSs 4 ask for an increase.
Power control of transmissions from the BTS 4 to the MS 10 (downlink power control) in IS95 is achieved as follows. When the MS 10 receives a downlink signal from a BTS 4 (or from each of a plurality of BTSs 4 in soft handoff operation) via a traffic channel (TCH), the FER of that signal is calculated by the MS 10 this reflects the degree to which the traffic-channel signal has been corrupted by, for example, noise. This FER is then relayed by the MS 10 to the BTS 4 which transmitted the downlink signal concerned, and the BTS 4 uses this FER to decide whether to make any change to its downlink transmission power.
The soft hand-off system described above is effective in improving signal transmission between the MS 10 and the network when the MS 10 is located in regions of cell overlap near the boundaries of the individual cells. Signal quality in these regions when using a single BTS 4 may be relatively poor, but by making use of more than one BTS 4 the quality may be substantially improved.
However, the IS95 soft hand-off system has the disadvantage of increasing signal traffic (xe2x80x9cbackhaulxe2x80x9d) in the fixed network 5 since it is necessary to transmit signals carrying the same data and/or control information between the BSC 6 and every BTS 4 involved in the soft hand-off for both the uplink and downlink cases described above. This duplication of information is undesirable for two main reasons. Firstly, it leads to more traffic congestion in the fixed network. Secondly, higher costs are experienced by the mobile service provider (and consequently the mobile service user), who may not own the fixed network infrastructure.
Therefore it is desirable to provide an improved soft hand-off method capable of affording the usual benefits of soft hand-off while at the same time reducing the load on the fixed network.
According to a first aspect of the present invention there is provided a cellular mobile communications network including: a mobile station; a plurality of base transceiver stations (base stations), each for receiving uplink signals from the mobile station; and a base station controller connected to the base transceiver stations for receiving there from such uplink signals; wherein the mobile station is operable, during a soft hand-off operation involving more than one of the base transceiver stations of the network, to include, in one or more such uplink signals transmitted thereby, respective signal measures for all of the base transceiver stations involved in the operation, each signal measure serving to indicate the performance of a communications channel between the mobile station and the base transceiver station concerned; and at least one of the base transceiver stations includes a processing unit operable, when that station is involved in such a soft hand-off operation, to determine, based on an assessment of the signal measure(s) for one or more of the other base transceiver stations involved in the soft hand-off operation, not to forward to the said base station controller means such an uplink signal received from the mobile station.
According to a second aspect of the present invention there is provided a mobile station, for use in a cellular mobile communications network, including: a transmitter for transmitting uplink signals to a base transceiver station of the network; and a selection processor connected to the transmitter means and operable, during a soft hand-off operation involving a plurality of such base transceiver stations of the network, to cause the transmitter to include, in one or more of the uplink signals, respective signal measures for all the base transceiver stations involved in the operation, each such signal measure serving to indicate the performance of a communications channel between the mobile station and the base transceiver station concerned.
According to a third aspect of the present invention there is provided a base transceiver station, for use in a cellular mobile communications network, including: a receiver for receiving uplink signals from a mobile station of the network, one or more of which uplink signals includes, when the mobile station is engaged in a soft hand-off operation involving the base transceiver station and at least one further base transceiver station of the network, respective signal measures for all the base transceiver stations involved in the operation, each signal measure serving to indicate the performance of a communications channel between the mobile station and the base transceiver station concerned; and a soft hand-off controller operable, when the claimed base transceiver station is involved in such a soft hand-off operation, to determine, based on an assessment of the signal measure(s) for one or more of the other base transceiver stations involved in the operation, not to forward to the base station controller of the network such an uplink signal received from the mobile station.
According to a fourth aspect of the present invention there is provided a soft hand-off control method for use in a cellular mobile communications network, wherein: when a soft hand-off operation involving more than one base transceiver station of the network is being performed, a mobile station of the network includes, in one or more uplink signals transmitted thereby, respective signal measures for all the base transceiver stations involved in the operation, each signal measure serving to indicate the performance of a communications channel between the mobile station and the base transceiver station concerned; and in at least one of the involved base transceiver stations, the signal measure(s) of one or more of the other base transceiver stations involved in the operation is/are assessed and a determination is made, based on the assessment, whether or not to forward to a base station controller of the network an uplink signal received from the mobile station.
According to a fifth aspect of the present invention there is provided a cellular mobile communications network including: a mobile station; a plurality of base transceiver stations, each for transmitting downlink signals to the mobile station and for receiving uplink signals from the mobile station; and a base station controller connected to the base transceiver stations for applying thereto such downlink signals; wherein the mobile station is operable, during a soft hand-off operation involving more than one of the base transceiver stations of the network, to produce respective signal measures for all the base transceiver stations involved in the operation, each signal measure serving to indicate the performance of a communications channel between the mobile station and the base transceiver station concerned; and the network including a base transceiver station selector, operable to employ the produced signal measures to determine which of the base transceiver stations involved in the operation should be used to transmit a subsequent one of the said downlink signals to the mobile station, and to cause that subsequent downlink signal to be transmitted by the base station controller only to the determined base transceiver station(s).
According to a sixth aspect of the present invention there is provided a mobile station, for use in a cellular mobile communications network, including: a transmitter for transmitting uplink signals to a base transceiver station of the network; and a selection processing unit connected to the transmitter and operable, during a soft hand-off operation involving a plurality of such base transceiver stations of the network, to produce respective signal measures for all the base transceiver stations involved in the operation, each such signal measure serving to indicate the performance of a communications channel between the mobile station and the base transceiver station concerned, and also operable to employ the produced signal measures to determine which of the involved base transceiver stations should be used to transmit a subsequent downlink signal to the mobile station, and to cause the transmitter to include, in such an uplink signal transmitted thereby, a base transceiver station selection message identifying the determined base transceiver station(s).
According to a seventh aspect of the present invention there is provided a base station controller, for use in a cellular mobile communications network to apply downlink signals to a plurality of base transceiver stations of the network, including: a receiver for receiving uplink signals from one or more of the base transceiver stations, at least one of which uplink signals includes, when a mobile station is engaged in a soft hand-off operation involving more than one of the base transceiver stations of the network, a base transceiver station selection message identifying which of the involved base transceiver stations should be used to transmit a subsequent one of the downlink signals to the mobile station; and a soft hand-off controller operable to receive the uplink signal including the base transceiver station selection message and to transmit the subsequent downlink signal only to the determined base transceiver station(s) identified in the message.
According to an eighth aspect of the present invention there is provided a soft hand-off control method for use in a cellular mobile communications network, wherein: when a soft hand-off operation involving more than one base transceiver station of the network is being performed, a mobile station produces respective signal measures for all the base transceiver stations involved in the operation, each such signal measure serving to indicate the performance of a communications channel between the mobile station and the base transceiver station concerned; and the produced signal measures are employed to determine which of the involved base transceiver stations should be used to transmit a subsequent downlink signal to the mobile station, and the subsequent downlink signal is transmitted by a base station controller of the network only to the determined base transceiver station(s).
The signal measures can be any suitable measure of the communications-channel performance between the mobile station and the base transceiver stations, for example signal strength measures (received signal strength in terms of power or amplitude or quality measures (frame error rate, signal-to-interference ratio, etc), or a combination of both strength and quality.
In preferred embodiments of the first to fourth aspects of the present invention the signal measures are respective power control bits received by the mobile station from the base transceiver stations involved in the soft hand-off operation. These power control bits indicate whether or not the mobile station is to increase or decrease its uplink transmission power to the base transceiver station and therefore serve conveniently as measures of the uplink channel performance channel between the mobile station and each base transceiver station.
In a further embodiment a cellular mobile communications network where a mobile station is capable of receiving a downlink signal from each of a plurality of base stations and transmitting an uplink signal to the plurality of base stations through a wireless channel, the network comprising: a selector unit for producing a measure of signal quality of the downlink signals from the plurality of base stations to the mobile station and for selecting a base station from which the downlink signal shows a preferred signal quality; a transmitter for transmitting the uplink signal indicating the selected base station among the plurality of base stations for subsequent communication with the mobile station; and a processing unit for processing the uplink signal to identify the selected base station, from among the plurality of base stations.
In this embodiment of the cellular mobile communications network, the mobile station may include the selector and transmitter. The transmitter may be operable to include an identification of the selected base station in the uplink signal. Further each base station includes the processing unit. The signal quality of the downlink signals from the plurality of base stations to the mobile station is represented by signal strengths of the received downlink signals.